We've Taken a Likin' to Lichen

Have you seen this little, bright, yellow-green bushy thing in the Sierras?

This is a wolf lichen. It is very common in the pine forests of the Sierras (and some other places too) - found on the tree trunks or fallen onto the ground. This particular lichen has been used as a source of yellow dye. Mildly poisonous, it has been used by indigenous peoples to poison arrowheads for hunting. Recently, wolf lichen has been the subject of scientific discovery, so keep reading to learn more!

["Brown-Eyed Wolf Lichen" by pellaea is licensed under CC BY 2.0 [Creative Commons]

My deep appreciation for lichens began long before I really understood what they were. I was traveling in Australia, and spent a couple months in eastern Tasmania, where a bright orange lichen lives in a thick band on the rocky coast. A fellow traveler I met there described it as dreamy.

[Photo taken by Jill Katz in Tasmania, Australia.]

At this time, I think all ​I knew was that lichens are crusty things on rocks. But truly, they are so much more!

As Bill Bryson wrote in A Short History of Nearly Everything, “Consider the Lichen. Lichens are just about the hardiest visible organisms on Earth, but the least ambitious.”

Lichens can be found pretty much everywhere! From hot and arid deserts, to wet rainforests; from sunny coasts, to high elevation and frigid, alpine zones. They grow on rocks, trees, other plants, other lichens, buildings, deserted cars, and even some animals. They grow in different structures, including fruticose (stringy/bushy, like wolf lichen above), crustose (crusty, often flat on rocks like on the coast of Tasmania), and foliose (leafy, like the lungwort lichen pictured below).

"Lungwort!" by Ken-ichi is licensed under CC BY-NC 2.0

Lungwort, a foliose (leaf-like) lichen. I saw this one frequently in western Washington.

But why should we care about lichens?

Lichens play an important role in ecosystems. They are involved in soil production and nutrient cycling; they can be used in bird nests, for insect camouflage, and as a food source for animals (some are edible for people, too!). People also have used many varieties of lichens for dyes and medicinal purposes.

Lichens can tell us a lot about an environment and its history. Many lichens are vulnerable to air pollution, so a change in their presence is an indicator of environmental conditions. (How often do you notice lichens in cities?) Many lichens have a known growth rate (for some, it’s less than 1mm in radius per year!) so measuring lichens can help historians to date events.

And there is much more we can learn from them.

[How many different kinds of lichens do you see? Photo taken at Huffaker Hills]

So what is a lichen?

*Describing a lichen accurately is no simple task. Prepare yourself for a bit of scientific language. But I will explain everything.*

Lichens are symbionts, they are structures of multiple species co-existing. This is also called a symbiotic relationship. These species include fungi and photobionts, species that produce their nutrients from sunlight (but these species are different from plants!). There are two identified kinds of photobionts in lichens: algae and cyanobacteria, but here, I’m just going to call them all ‘PBs’. There are different kinds of fungi, too, but I’ll touch a bit more on this later.

Note: Mosses are sometimes confused with lichens. They may share habitats, bushiness, green color, and even common names (e.g., “reindeer moss” is a lichen). However, a single moss is one plant species, and a single lichen includes multiple species of fungi and non-plant photobionts.

[Moss is growing as a thick blanket around some tree trunks. Lichen is hanging from the branches. A deer is posing. Photo from Olympic National Park, an excellent destination for both moss and lichen appreciation.]

How do they work as symbionts? Perhaps the most famous explanation comes from lichenologist Trevor Goward, who said, "Lichens are fungi that have discovered agriculture." The fungal cells take hold of the PB cells and make the PB cells “leaky” so they can easily obtain the nutrients the PBs produce. While the PB cells may be protected from drying out, it is thought they give to- more than they receive from the fungi. Therefore, this relationship is not thought to be mutualistic, equally benefitting all members.

[Lichens can be found just about anywhere. I photographed this lichen on a fence in my neighborhood.]

The structure of these symbiotic collections of cells is fascinating. When I first learned of this structure, I immediately thought, lichens are like brains!! Their outer layer is called the cortex (like the outer surface of the brain) and a loosely packed layer of hyphae, filaments from the fungal cells, is called a medulla (like a very important part of your brainstem). While the cortex and medulla of the brain serve different functions, just like in a brain, each part of the lichen serves a unique and important role in relation to all of the other parts. Also just like the brain, scientists have much more to discover!

[Graphics by Emily Eiswert, Restoration Tech at the Proposed Nature Study Area. Foliose and crustose lichens are often structured in clear layers, though they might not be as distinct as shown here. The outer layer is the cortex, which is composed of fungal cells. The cortex might provide a unique pigment to the lichen. It also may be mostly transparent, so the visible color of the lichen is from the below algal layer (composed of non-fungal, algal or cyanobacterial cells). The medulla is section is mostly composed of hyphae- which are part of the above fungal cells. The hyphae are loosely packed in this layer. Some lichens have a lower cortex, similar to the upper cortex, and rhizines to attach to the substrate (whatever the lichen is growing on). But in crustose lichens, the hyphae attach to the substrate directly.]

Still Discovering...

Major lichen discoveries have been published in the last few years. In 2016, researchers in the U.S. found some varieties of lichens have DNA of not just the expected kind of fungus (of the phylum Ascomycota, if you’re curious), but additional DNA from yeasts (also a fungus, of the phylum Basidomycota). In just 2019, researchers found that our local wolf lichen has two different species of yeast! These findings are revolutionizing our ability to understand the complexity of lichens.

As we continue to learn more about lichens, we improve our ability to understand history, the environment, and how life works.

“Of what a strange nature is knowledge! It clings to a mind when it has once seized on it like a lichen on a rock.”

- Mary Shelley, Frankenstein